The present invention relates to an amplifier with a power supply for amplifying several audio signals in parallel, as known for example from integrated stereo amplifiers or surround sound amplifiers. High quality audio amplifiers have usually an output power of about 100 Watts or more per channel, for driving respective loudspeakers. In case of an surround sound amplifier, which is known for example as Dolby-Digital amplifier or Home-Cinema receiver, the amplifier has six or more separate audio channels in parallel, and therefore the power supply of the amplifier has to handle a very high total power and special means have to be provided for the amplifier to avoid over-current conditions and over-temperature conditions. Current standards, which a surround sound amplifier at present performs, are for example Dolby Pro-Logic, Dolby Digital AC-3 and DTS, with 5.1 multi-channel decoding.
He is an object of the present invention is therefore, to provide an amplifier for amplifying at least one audio signal, which comprises protection means for avoiding an overload condition, especially with regard to the output stages.
The amplifier according to the invention, for example a surround sound amplifier, comprises a power supply, a current sensing element for sensing a current in the current path of the power supply, a detecting circuit responsive to the current sensing element, and voltage reducing means for reducing the signal amplitude of at least one of the audio signals in response to the detection circuit.
The detection circuit comprises in a preferred embodiment a threshold circuit, and when the current in the current path of the power supply is above a predefined threshold, the detection circuit together with the voltage reducing means reduce at least one of the audio channels. This threshold is adjusted such, that the current drawn from the power supply is below an overload condition of the power supply.
The current sensing element is for example a current sensing resistor and the detecting circuit is a voltage detection circuit in parallel to this resistor. The voltage reducing means comprise advantageously two diodes for each channel, via which a positive and a negative voltage limit is applied to the signal path of the audio signals. When the current from the power supply is too high, the voltage above the current sensing resistor passes the predefined threshold, which causes the voltage reducing means to apply a positive and a negative voltage limit for the audio signals. The higher the voltage above the current sensing resistor is, the lower will be the voltage limits.
The voltage limits may be applied advantageously to the signal path of the audio signals between the preamplifiers and the output power amplifiers via the diodes. With this arrangement an existing design of a surround sound amplifier does not have to be modified.
The power limiting circuit as described has the following advantages: It provides an over-current protection for the transformer for avoiding a respective overheating of the transformer. Further, it avoids an overheating of the transistor stages in the output power amplifiers, which need therefore no temperature protection.